A simple method of propagating an electrical signal is to use a single-ended line. However, single-ended lines tend to be vulnerable to noise effects. This can be a particular problem in low-voltage circuits, especially in a crowded and compact electrical environment such as a mobile phone or other portable radio device. In such devices a great deal of functionality is required in as small a volume as possible which can lead to problems with electromagnetic compatibility.
Differential lines may be used instead of single-ended lines. Signals transmitted on single-ended lines are simply referenced to ground with any noise picked up during transmission superimposed on them. Differential lines are pairs of lines with one carrying a positive version of the signal and the other a negative version. Their outputs are then subtracted from one another so that noise effects affecting both lines are cancelled out and the overall signal to noise ratio, even for noise only affecting one of the two lines, is reduced.
Although differential lines are generally an improvement over single-ended lines, some problems remain. Large voltage swings can occur on long lines, resulting in electrical perturbation and power loss. In addition, due to the spatial separation between the two lines, some noise sources (e.g. magnetic aggression and parasitic electrical effects) are likely to affect one line to a greater degree than the other. Asymmetrical effects like this are not cancelled when the differential signals are subtracted.
The latter of the problems mentioned above impedes the use of differential lines for propagation of in-phase and quadrature signals. These are two versions of the same local oscillator signal, with a phase difference of ninety degrees. Such signals are commonly used in the coding and modulation/demodulation and decoding of radio messages in the technique known as quadrature amplitude modulation. To be useful for these purposes the integrity of the ninety degree phase relationship of the in-phase and quadrature signals must be maintained. However, in transceiver chip implementations (which must be as small as possible) the in-phase and quadrature signals generally need to be transmitted with little spatial separation. If differential lines are used in this case, then the parasitic capacitance and inductance generated between lines has a greater effect on adjacent lines than non-adjacent lines, leading to degradation in the phase relationship.
The conventional method for propagating local oscillator signals from a phase locked loop to a transmit or receive chain on-chip is to transfer the local oscillator signals as differential voltage signals, typically rail-to-rail signals. This generates electrical perturbation on the silicon substrate and surrounding circuits. Moreover, in such an arrangement, buffers used to transfer the signal generated by the local oscillator to a power amplifier need to have a very high reverse isolation to avoid the quality of the local oscillator being corrupted due to the signal present on the transmitter mixer. (The transmitter mixer uses the local oscillator signals and a base-band signal to generate the radio frequency signals for application to the power amplifier and antenna.) This is called local oscillator pulling.
What is needed is an apparatus for propagating local oscillator in-phase and quadrature signals with high reverse and external isolation, high common mode rejection, and electrical and electromagnetic perturbation as low as possible.